Add safe custom JSON Agent Channel runner

[mimeding]

Summary

• Adds a configuration-only Custom JSON runner for Agent Channels with method, host, IP, header, body, query, redirect, and response-size safety controls.
• Adds redaction, write gates, idempotency handling, cancellation reporting, and standard action mapping.
• Rejects non-JSON templated bodies and percent-encodes templated query values to prevent parameter injection.

Validation

• swift test --package-path Packages/OsaurusCore --filter CustomJSONAgentChannelRunnerTests
• swift test --package-path Packages/OsaurusCore --filter DiscordConnectionTests
• bash scripts/i18n/check.sh
• swiftlint lint --quiet Packages/OsaurusCore/Services/AgentChannel/AgentChannelCustomJSONRunner.swift Packages/OsaurusCore/Tests/AgentChannel/CustomJSONAgentChannelRunnerTests.swift Packages/OsaurusCore/Tests/Discord/DiscordConnectionTests.swift
• git diff --check
• External review pass completed; actionable findings were incorporated before publication.
• GitHub checks are green.

Notes

• Ready for review as the configuration-only custom Agent Channel execution path. This branch does not change model behavior, prompts, or default tool exposure.